With the continuous casting in common practice at present, supply of a high temperature slab to a heating furnace for rolling has been a great question to be solved in the aspect of energy saving. Because of this, in the continuous casting operation, necessity has been voiced for high speed pouring and supply of a slab to a rolling section for a short period of time due to quick detection of surface defects. However, since the pouring rate is high during high speed pouring, the thickness of a solidified shell formed in the slab is small, and there is a possibility of occurrence of a so-called breakout, that is, the solidified shell may be broken off when the thin portion of the solidified shell reaches the lower end of a continuous casting mold (hereinafter referred to as the "mold") within the mold. However, occurrence of the breakout has not heretofore been accurately predetected. Hence, in order to avoid the breakout, the pouring rate is reduced beyond necessity. Or, after the breakout has occurred, an operation stop for several hours has been necessitated. On the other hand, surface defects such as longitudinal surface cracks are mainly caused due to the fact that the extracted heat value is varied by ununiformity of the mold powder flowing into a space between the mold and molten steel (slab), and particularly, the local decrease or increase thereof, whereby the formation of the solidified shell becomes ununiform. However, since surface defects have heretofore been detected through (1) a crack check and trimming after rolling, (2) a visual inspection after cooling of the slab, or (3) an inspection after the withdrawing and cooling of the slab and the like, such disadvantages have been presented that, (1) the process is carried out after the defects are detected, necessary feedback steps cannot be taken during the pouring operation, and thus the yield is lowered, (2) the slab need to be cooled, a unit consumption of the heating furnace is increased, or (3) defects cannot be fully detected.
As a method of predetecting the aforesaid breakout, there has heretofore been proposed one in which a distortion of a main shaft in an oscillation mechanism for oscillating a mold during pouring is measured to predetect a restraining breakout. However, this method is disadvantageous in that a breakout at a distortion of a low value cannot be detected and this method is applicable only during steady pouring (at a constant drawing rate).
There has been proposed a method, in which an oscillation waveform of the oscillation mechanism is measured and an abnormal waveform is detected to thereby predetect a breakout. However, this method is disadvantageous in that a fine variation cannot be obtained from the oscillation system itself.
Further, there has been proposed a method in which a bulging value of a portion bulged directly downwardly from the slab is measured to predetect a breakout. However, this method is statistical one, can indicate only a probability of occurrence of a breakout, and cannot directly detect the behavior in the mold.
On the other hand, it is a well known fact that all of the breakouts and surface defects as described above closely relate to a contacted state between the mold and the slab (that is, the heat extraction). It is conceivable that a breakout or a crack of the slab can be predetected through the measurement of the extracted heat value or the distribution thereof, because heat transfer to the mold is high in value through a thin portion of the solidified shell, or the distribution of the extracted heat value becomes ununiform when the contacted state between the mold and the slab becomes ununiform, for example. In consequence, there have heretofore been practised that, for example, as shown in FIG. 1, holes 11b are formed in the bottom portion of cooling water paths 11a provided on outer side surfaces of mold shell plates 11 forming a mold 10, thermocouples 12 are embedded in the aforesaid holes 11b, and a heat flux is determined through calculation of a temperature gradient detected from outputs of the thermocouples embedded at two points spaced apart from each other in the direction of depth so as to detect the heat extraction. However, with this method, not only thermal agitation occurs due to the embedding of the thermocouples 12, but also the thermocouples need to be embedded at accurate positions because, if the embedded positions are shifted by 1 mm for example, then there occurs an error of 5.degree. to 10.degree. C., so that great difficulties are encountered in the embedding operation. Furthermore, when an extracted heat value Q is calculated from detected temperatures T.sub.1 and T.sub.2 from the two thermocouples, an interval d across the embedded positions and a thermal conductivity .lambda. of a mold 10 in accordance with the following equation, errors may be caused to the detected temperatures T.sub.1 and T.sub.2 due to the thermal agitation, and moreover, an error may be caused to the interval d due to an error in the embedded position, to thereby easily cause errors. EQU Q=.lambda.(T.sub.1 -T.sub.2)/d (1)
Further, it is impossible to directly indicate and record a heat flux. Furthermore, the variations in value of the outputs from the thermocouples at the time of breakout or occurrence of surface defects are comparatively low as shown in FIG. 2 (the case of breakout), a change in temperature increase such as 5.degree. to 10.degree. C. in short time interval must be inspected in order to sense a breakout for example, so that difficulties are encountered in determining the breakout. Further, with the thermocouples, exact numerical values including a change in temperature at the time of a breakout, a change in temperature at the time of occurrence of surface defects and the like cannot be grasped due to factors such as a change in the thickness of mold caused by wear of the slab, errors in the embedding of the thermocouples themselves and the like. In the case of occurrence of a longitudinal crack, if a variation in numerical value is small, then the occurrence of the defect cannot be detected. Further, such disadvantages have been presented that the embedding of the thermocouples in holes formed in the mold side plate shortens the service life of the mold, reinstalment is difficult to conduct and so forth.
On the other hand, it is very important for controlling the surface quality of a slab to control the behavior of heat extraction of the mold. In consequence, there has heretofore been developed a semi-automatic supply system capable of mechanically supply an input of the mold powder, which has been manually preset, so as to quantitatively grasp the input of the mold powder rendering influences onto the the behavior of heat extraction as commensurate to the progress of the continuous casting. However, since the presetting of the amount of supply of the mold powder, scope of supply, brands, mixture ratio and the likes have heretofore been conducted on the basis of the results of the visual determination of the dissolved condition of the powder through the observation and the like of the molten steel surface made by an operator, such disadvantages have been presented that local changes of the powder flow-in conditions in the mold cannot be sensed, a necessary feedback step for the quality of slab is belated, the extracted heat value is varied due to ununiformity in the amount of the mold powder flowing into a space formed between the mold and the molten steel (slab), particularly, the local decrease or increase, whereby the formation of the solidified shell becomes ununiform, so that surface defects such as a longitudinal crack and the like are caused to the slab, to cite the extreme case, a breakout occurs.
Further, in the continuous casting, a solidified shell is contracted during pouring. In consequence, shell plates on the short sides, which form the mold, are tapered, so that the solidified shell and the shell plates of the short sides can be brought into full contact with each other. However, in case the taper value of the shell plates of the short sides is small, the solidified shell and the mold are in insufficient contact with each other, whereby the cooling is not satisfactorily conducted and a slab goes out of the mold before the thickness of the solidified shell is developed, thus presenting a danger that cracks due to the static pressure of molten steel occur or the solidified shell is broken off to generate a breakout. On the contrary, in case the taper value of the shell plates of the short sides is excessively large, the solidified shell and the mold are violently brought into contact, thereby presenting a possibility that an excessive deforming stress acts on the solidified shell to break the same off or wear of the mold is intensified due to friction between the solidified shell and the mold, thus resulting in shortened service life of the mold. In consequence, the taper value has heretofore been set on the basis of experience prior to the start of pouring depending on the grade of steel, pouring rate and the like. After the start of pouring, the set taper value is changed in accordance with changes of the grade of steel, pouring rate and the like in the course of pouring, and thus, the operation is continued. However, the taper value set on the basis of the experience depending on the grade of steel, pouring rate and the like has not been set on the basis of direct study on the degree of contact between the solidified shell and the mold due to delicate variations in the mold powder, grade of steel and pouring rate, whereby there have occurred some cases where the set taper value is not suitable, thus causing surface defects such as side surface cracks, minute longitudinal cracks and the like of the slab.
The present invention has been developed to obviate the above-described disadvantages of the prior art and has as its object the provision of method of controlling continuous casting equipment, capable of easily and reliably predetecting occurrence of a breakout or a crack of a slab with high sensitivity throughout all of the operating conditions, thereby reliably preventing occurrence of a breakout or a crack.
Further, the present invention has as its object the provision of method of controlling continuous casting equipment, wherein heat flux meters capable of directly measuring heat fluxes are provided in suitable states, measuring a heat extraction of the mold with high accuracy and preventing the service life of the mold from being shortened.
Further, the present invention has its object the provision of method of controlling continuous casting equipment, wherein the heat flux meters can bes easily provided.
Furthermore, the present invention has its object the provision of method of controlling continuous casting equipment, capable of accurately measuring heat flux waveforms or heat flux values.
Furthermore, the present invention has as its object the provision of the method of controlling continuous casting equipment, wherein an optimum taper value can be quickly and precisely obtained as commensurate to changes in the contacted state between the solidified shell and the mold during operation, so that a breakout, a crack of the slab and a wear of the mold can be reliably prevented from occurring.